Cystic fibrosis (CF) patients are frequently underweight. Nutritional rehabilitation has been shown to improve pulmonary function in CF, and liquid dietary supplementation are frequently used to improve the caloric intake in these patients. Several studies have shown hyperglycemia and glycosuria associated with nighttime enteral nutritional supplementation. However, very little is known about the effect of nutritional supplementation on pancreatic beta cell function. CF patients have decreased insulin secretion from beta cells, although the age of onset of this beta cell dysfunction is unknown. We hypothesize that the nighttime enteral feeding causes acceleration of beta cell dysfunction and leads to earlier development of cystic fibrosis related diabetes mellitus.